CAPILLARY ATTRACTION. 



145 



What is a 

 " Kope Pump ?" 



Fig. 138. 



when they apply them to the surface of water, mstead of sinking in it, they 

 produce depressions upon it. 



For a like reason, water will not flow through a fine sieve, the wires of 

 which have been greased. 

 ,^ .„ 330. A liquid will not wet a solid when the 



When will a -*• . 



liquid fail to force of adhesion developed between the par- 

 wet a Golia ' , . 1 „ . . . 



ticlcs of the liquid and the surface of the solid, 

 is less than half the cohesive furce which exists hetweea 

 the particles of the liquid. 



331. The fact of the strong adhesion 



wliich exists between water and the 



fibers of a rope, has been taken ad- 

 vantage of in the construction of a kind of pump, called 

 the " Rope," or " Yera's'' Pump, Fig. 138. It con- 

 sists of a cord passing over two wheels, a and b, the 

 lower one of which is immersed in water. A rapid 

 motion is given to the wheels by means of the crank 

 d, and the water, by adhering, follows the rope in its 

 movements, and is discharged into a receptacle above. 



„^ , , Illustrations of capillary attraction 



AVhat are fa- ' •' 



miliar iiiustra- are most famUiar m the experience of 



ISattracti'o^? everyday hfe. The wick of a lamp, 

 or candle, lifts the oil, or melted grease 

 which supphes the flame, from a surface often two or 

 three inches below the point of combustion. In a 

 cotton-wick, which is the material best adapted for this purpose, the mi- 

 nute, separate fibers of the cotton themselves are capillary tubes, and the in- 

 t'^rstiees between the filaments composing the wick are also capillary tubes ; 

 in these the oil ascends. The oil, however, can not be lifted freely beyord a 

 certain height by capillary attraction : hence, when the suriace of the oil id 

 low in the lamp, the flame becomes feeble, or expires. 



If the end of a towel, or a mass of cotton thread, be immersed m a basin of 

 water, and the remainder allowed to hang over the edge of the basin, the 

 water wiH rise through the pores and interstices of the cloth, and gradually 

 wpt the whole toweL In this way the basin may be entirelj- emptied. 



If sand, a lump of sugar, or a sponge, have moisture beneath and slightly 

 in contact with it, it will ascend through the pores by the agency of capillary 

 attraction in opposition 1o gravity, and the entire mass will become wet. 



The lower story of a hou'^e is sometimes damp, because the moisture of the 

 ground ascends through the pores of the materials constituting the walls of 

 the building. "Wood imbibes moisture by the capillary attraction of its pores, 

 and expands or swells in consequence. This fact has been taken advantage 

 of for splitting stones ; wedges of dry wood are driven into grooves cut in the 

 stone, and on being moistened, swell with such irresistible force fia to split 

 the block in a direction regulated by the groove. 



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